Is this behavior controlling or prudent?

I have been dating this girl now for three, count em, three years. i am eighteen and she is nineteen and we still havent had . She has been acting like she wants to have for at least the last year, but every time i ask her, she says no. Its very frustrating and it makes me want to give up. Now she says that she wont have with me until i agree to marry her. what kind of guy my age has to worry about marriage agreements? what guy my age is still a virgin? Shes a self-declared prude and she seems proud of it. I love her and she loves me, but i have needs and i am starting to want to break it off forever. What should i do? What do you think of her behavior?